Superconductors are materials that allow the flow of electric current without resistance at or below some critical transition temperature. Recent research has led to the development of superconductors with transition temperatures about the temperature of liquid nitrogen (77.degree. K.). Research to identify superconductors with even higher transition temperatures is continuing.
Because of the unique electrical properties exhibited by superconductors, many believe that they possess some of the most exciting prospects for engineering development and application in this century. Such potential commercial applications range from, for example, the production of highly efficient power lines for use in transmitting electricity from power plants to distant cities to the development of high speed levitating trains.
Several different approaches have been suggested for processing superconducting materials for widespread commercial application. One such process developed at Northwestern University is known as sinter-forging. In sinter-forging, the ceramic superconducting material is fired under pressure in an effort to maximize its density in a monolith of desired shape. While the pure ceramic product is superconducting, it is also very brittle and, thus, not able to support any significant stress. This makes it impractical for most potential applications.
Superconducting ceramic matrix-metal composites have been suggested as a possible solution to the brittleness problem. In particular, it is believed metals such as copper, silver, gold and alumninum add strength to the superconducting ceramic. In addition, these metals do not significantly impair the efficient operation of the superconducting ceramic since these metals also possess low resistivities. Processing such a composite, however, creates a number of significant problems including the maintenance of the superconducting behavior of the final product.
In this regard, it should be noted that the sinter-forging process discussed above is not adapted to produce the desired metal superconducting ceramic matrix composite. More specifically, the relatively high temperatures and long fabrication times characteristic of this method tend to cause rapid oxidation of the metal in the composite and oxygen depletion of the superconducting lattice. This leads to an unacceptable degradation of superconducting properties.
In another process known as shock-wave consolidation, superconducting ceramic and a metal is placed into a die. The ceramic and metal is then consolidated into a contiguous monolith by dynamic compaction using shock waves generated by explosives. Because of short fabrication times and generation of relatively little heat, the degradation problems associated with the sinter-forging process are generally avoided. As such, composites of superconducting ceramic matrix and metal can be effectively fabricated with this method. Still, however, the method does have a number of shortcomings.
In particular, there is the danger associated with any use of explosives. Further, the size of the explosion required for consolidation is proportional to the size of the monolith to be produced. Thus, the size of the composite monolith that may be produced by this method is limited by the size of the explosion that can be safely controlled. Additionally, it should be appreciated that the rate of processing cannot be effectively controlled. Finally, only relatively simple shapes can be produced. A need is, therefore, identified for an improved method of processing both superconducting ceramic and metal--superconducting ceramic matrix composites into relatively complex monolithic shapes.